The subject matter disclosed herein relates to turbine systems, and more particularly to a turbine blade monitoring arrangement, as well as a method of manufacturing the turbine blade monitoring arrangement.
Efficiency of a gas turbine engine is impacted by clearance between an outer radial tip of rotor blades and the surrounding stationary structure, which is referred to herein as “tip clearance.” Tighter clearances decrease the leakage flow around the rotor blades, which improves engine efficiency. However, tighter tip clearances increase the risk that rotating parts will make contact with or rub against non-rotating parts during one of the engine's several operational modes, particularly considering that tip clearances generally vary based upon operating conditions. Primarily, this is due to the different thermal expansion characteristics of many of the engine components. Of course, having rotating and stationary parts rub or make contact during operation is typically undesirable because it may adversely affect various components or operating modes. In addition, rubbing may result in increased clearances once the event that caused the rubbing passes. On the other hand, the engine may be designed with more open clearances that decrease the likelihood of rubbing parts. However, this is undesirable because it generally allows for more leakage and thereby decreases the efficiency of the engine.
Maintenance of a gas turbine engine is generally planned around specific operation of the engine, as recorded in number of starts and hours of operation. Sensors could be employed to measure the condition of the gas turbine components to determine when maintenance is required based on a measured hardware condition. Creep of turbine hardware over time is an indicator of when hardware maintenance is required. Turbine hardware condition could be used to delay planned maintenance or schedule maintenance earlier to prevent a possible failure.
Gas turbine engines may employ active clearance control systems to manage the clearance during a myriad of operating conditions so that a tight, non-rubbing clearance is maintained. It will be appreciated that these systems need regular, updated, and accurate tip clearance data to realize the full benefit of the clearance control system. Conventional measurement systems measure tip clearance with proximity sensors positioned in the hot-gas path. Typically, these probes are positioned directly over the rotor blades and measure the distance between the probe and the blade tips of the rotor blades as the blades pass. The downside of positioning the sensors in this manner is that the sensors are exposed to the extreme temperatures of the hot gas path. Sensors that are able to withstand these conditions while providing accurate measurements are expensive. Even so, because of the extreme conditions of the hot-gas path, these sensors can have short lifespans, which increase costs and maintenance requirements. Also, these sensors typically require a supply of cooling air, which may be bled from the compressor or supplied from an auxiliary source. It will be appreciated that providing cooling air in this manner adds complexity to engine systems and decreases the efficiency of the engine.
As an alternative to positioning the sensors within the hot gas path, sensors may be positioned in remote locations outside of the hot gas path to measure distances to other turbine components. These measurements are then employed to estimate tip clearance indirectly based on calculations employing the mechanical and thermal displacements of the relevant turbine components and measurement data.